1. Field of the Invention
The present invention relates to ultrasonic diagnostic apparatus for displaying a moving body in color and more, particularly relates to ultrasonic analysis equipment that provides a frequency and a sign output corresponding to blood flow speed and direction by means of a color flow mapping analyzer.
2. Description of the Prior Art
Ultrasonic diagnostic apparatus utilizing a color flow mapping analyzer applies ultrasonic waves onto a human body and measures the frequency difference between the applied and reflected waves to determine the speed of blood flow. Thus, the ultrasonic diagnostic apparatus permits the easy measurement of cardiac blood flow.
A color flow mapping analyzer incorporated in the ultrasonic diagnostic apparatus orthogonally detects a received signal determined from a reflected wave by the sent signal corresponding to an applied wave and the signal whose phase is offset from the sent signal by 90 degrees to get two output signals whose phases are offset by 90 degrees from each other. After the removal of the unnecessary signals such as those from the heart wall, an auto-correlation coefficient is determined from the two output signals, X and Y. The absolute frequency value from the determined auto-correlation coefficient is output, with f, s and p denoting frequency, the positive or negative sign of the frequency from the auto-correlation coefficient, and the strength of the two output signals, X and Y, respectively. Thus, frequency f output from the color flow mapping analyzer reflects the speed of blood flow, sign s indicates its direction and strength p represents the strength of the reflected wave.
In the ultrasonic diagnostic apparatus utilizing the color flow mapping analyzer, a positive sign s output therefrom indicates the shade corresponding to frequency f, e.g., blue, and a negative sign s indicates the shade corresponding to frequency f, e.g., red to provide images covering the distribution of blood flow. If the strength p output from the color flow mapping analyzer is below a given threshold, the value of the frequency f is replaced by 0, since the SN ratio of the reflected wave is degraded. Thus, the effects of noises can be eliminated.
As a prior art of the ultrasonic diagnostic apparatus, the Japanese Patent Application, No. 48233-1987 (the Japanese Patent Publication Open to Public Inspection, No. 270139-1987) is known.
The prior art is such that, if strength p output from the color flow mapping analyzer is below a given threshold, frequency f is replaced by 0; since a too high threshold set results in degraded image quality (black-holes appear in the image), however, a threshold is set within allowable measuring accuracy. Thus, frequency f output from the color flow mapping analyzer corresponding to strength near a threshold contains slight noises, making it impossible to completely eliminate its effects.
Contained noises lead to random values of frequency f and sign s, causing the partial inversion of values of sign s. A result is the occurrence of the problem that the blue color indicating that blood flows in a certain direction is mixed with red colors and images are disordered.
Such disorder may result in improper diagnosis in hospitals which use the ultrasonic diagnostic apparatus of the prior art.
A possible method to solve the disorder of images caused by the inversion of sign s by noises is such that signs s in a given area are averaged to compensate for local variations in sign s. Although, in general, low-speed blood flow encounters no reverse flow, however, the fact is observed that high-speed blood flow classified into turbulent flow involves local reverse flow. Thus, the compensation for image quality by averaging signs s also makes uniform local reverse flow caused by turbulent flow. Hence, such compensation cannot be applied actually.